Facilitative interactions on coastal dunes in response to seasonal weather fluctuations and benefactor size

Abstract

:We analyzed spatial plant-plant associations (during three seasons: nortes, dry, and rainy) and performed a field experiment on seedling survival, in order to test whether the occurrence of facilitation varied in response to seasonal weather fluctuations and to benefactor size. In a mobile dune system (central Gulf of Mexico) we focused on the endemic early colonizer shrub Chamaecrista chamaecristoides and two late colonizer bunch grasses. Total shrub cover and density were highest during the rainy period. Density of adult Trachypogon plumosus, but not that of adult Schizachyrium scoparium, was higher beneath the canopy of the shrub. No seasonal variations in spatial associations were observed. Density of grass seedlings decreased during the nortes beneath and beyond the canopy of C. chamaecristoides. Fitness parameters of both grasses improved beneath the benefactor plants, while sand accretion decreased. The number of grass individuals benefiting from facilitation increased with shrub size except for the largest shrubs, which had a reduced density of grasses. Temperature on the sand surface was lowest beneath the larger shrubs. Our results highlight the importance of different life stages at which facilitation may occur together with a varying effect of benefactor size in the overall role of facilitation in structuring communities.